Thermal Indicators

ABSTRACT

A thermal indicator using indicia formed by opaque ink applied to a thermal imaging material element is described. The ink is substantially the same color as the imaging element background and remains substantially hidden until the imaging element background changes color when heated. Time-temperature exposure indicating labels can be prepared using a printer with direct thermal type and ink jet type print heads.

RELATED APPLICATIONS

This application is a Divisional Application of, and claims priority to,U.S. application Ser. No. 11/365,190, filed Mar. 1, 2006, entitled“THERMAL INDICATORS,” which is hereby incorporated by reference hereinin its entirety for all purposes. This application is also related tothe following other Divisional Applications of U.S. application Ser. No.11/365,190, U.S. application Ser. No. ______.

BACKGROUND

Direct thermal printing is a recognized means of printing quietlywithout toners or inks. It is a relatively mature technology that hasbeen around for over forty years. Its use by retailers for printing ofcash register receipts, mailing labels, etc. is now commonplace.

In direct thermal printing, a print head selectively applies heat topaper or other sheet media comprising a substrate with a thermallysensitive coating. The coating changes color when heat is applied, bywhich “printing” is provided on the coated substrate. For dual-sideddirect thermal printing, the sheet media substrate may be coated on bothsides.

Time-temperature indicators using thermally sensitive color changematerials are well known. For example, indicator devices that relay ondiffusion of a dye through a polymer are described in U.S. Pat. Nos.6,214,623; 5,746,792; 5,057,434; and 4,212,153.

SUMMARY

A thermal indicator using indicia formed by opaque ink applied to athermal imaging material element is described. The ink is substantiallythe same color as the imaging element background and remainssubstantially hidden until the imaging element background changes colorwhen heated. Time-temperature exposure indicating labels can be preparedusing a printer with direct thermal type and ink jet type print heads.

Other features, advantages and variations of the invention will beapparent from the following description and the appended drawings andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a thermally sensitive label with hidden opaqueprinting before exposure to excessive heat.

FIG. 2 illustrates the thermally sensitive label of FIG. 1 followingexposure to excessive heat, where the opaque printing has been madevisible.

FIG. 3 shows schematically a direct thermal printer with an ink jetprint head.

DESCRIPTION

By way of example, various embodiments of the invention are described inthe material to follow with reference to the included drawings.Variations may be adopted.

FIG. 1 illustrates a thermally sensitive label 10 with hidden opaqueprinting 20 before exposure to excessive heat. The label 10 comprises asubstrate with one or more thermally sensitive coatings, e.g.,permitting direct thermal printing 30 on the label 10 in a manner wellknown in the art. The label 10 may also include printed material such asa logo 40 added by direct thermal printing, inkjet printing,lithographic process, flexographic printing or the like. Conventionaldirect thermal printing paper can be used for the exemplary labels 10.

In the example shown in FIG. 1, the initial background color of thelabel 10 is white. The hidden opaque printing 20 would match thisbackground color so as to be essentially invisible before the label 10is exposed to excessive heat. In this example the color of the opaqueprinting 20 would be white. The visibility of the hidden opaque printing20 in FIG. 1 is for illustration only, where again the opaque printing20 would be substantially invisible in actual practice against thebackground of the label 10. The hidden printing 20 could be added to thelabel 10 by inkjet printing, for example.

When the label 10 is exposed to excessive heat, the background color ofthe label 10 turns dark, as shown in FIG. 2, exposing the opaqueprinting 20 and rendering it visible, e.g., to reveal a message. Theillustrated label 10 in FIGS. 1 and 2 could be used, for example, forsafe guarding medication in pill bottles from excessive thermalexposure. The safe guard would be an integral part of the label on thebottle. In this embodiment the opaque printing 20 is a warning messagepreferably pre-printed on a white direct thermal label 10 using opaquewhite ink. The white on white printing is initially invisible. When thelabel 10 is exposed to excessive temperature the entire label images, orturns dark. The initially invisible white printing 20 becomes visible.This is shown in FIGS. 1 and 2. The activation temperature when thelabel 10 turns dark can be selected based on requirements for safeguarding particular medication in containers to which the label 10 isapplied.

The white warning message or printing 20 may be optimally placed on aportion of the label not thermally imaged by thermal printing 30 orprinted with logo 40. This is depicted in FIG. 1. However by adjustingthe opacity of the white ink it is possible to place the invisible print20 on areas 30 of the label 10 that are thermally printed if desired.This can be accomplished by adjusting the opacity of the white to allowthe thermally imaged areas 30 to appear gray through the whitepre-printing. As long as the thermal printing is sparse an observer willnot detect the hidden message 20 before heat activation.

The activation temperature for revealing the hidden message 20 can beadjusted by changing the thermal sensitivity of the label 10. Thesubstantially opaque white message 20 may be applied over a protectivelayer as desired. Alternatively, or in addition, a substantiallytransparent protective layer may be applied to the label 10 on top ofthe printing 20. The label 10 is not limited to white thermal paper andthe hidden message 20 is not limited to white opaque ink. Other colorsof paper and inks may be used. Similarly, the label 10 is not limited toblack thermal imaging, other imaging colors being possible.

In another application of a thermal label or thermal paper 10, thehidden message 20 could be used as a security feature. When the paper 10is thermally printed an area can be intentionally thermal printed toexpose the hidden print 20, authenticating the media.

The foregoing description above presents a number of specificembodiments or examples of a broader invention. The invention is alsocarried out in a wide variety of other alternative ways which have notbeen described here. Many other embodiments or variations of theinvention may also be carried out within the scope of the followingclaims.

As shown in FIG. 3, a printer 50 for printing of time-temperatureindicator labels 10 can be constructed using a direct thermal printingprint head 60 to print first indicia 30 on the labels 10, and an ink jetprint head 70 to print second indicia 20 on the labels 10, using wellknown print head technologies. The printer 50 includes a platen 80opposing the thermal print head 60 and the labels 10 are presented forprinting on a web moving through the printer 50 along a feed path 90.Such a printer 50 would comprise a supply of ink (not shown) for the inkjet print head 70, where the ink is of a color that substantiallymatches that of the thermal media elements 10 prior to activation due toexcessive heat.

1. A method of authenticating a thermal media imaging element includinga thermally sensitive coating on a substrate, where said thermallysensitive coating changes color upon exposure to heat, comprising:providing said imaging having a security feature including indiciaformed over said coating by opaque ink of a color substantially matchingthat of said imaging element, said indicia being substantially invisibleuntil said coating is exposed to heat; and heating said coating torender said indicia visible to authenticate said imaging element.
 2. Themethod of claim 1 in which said coating is heated by operation of adirect thermal printer to authenticate said imaging element.